Metal caps are used for sealing the open ends of containers such as bottles or wide neck jars and comprise a metal shell to which a lining layer has been added. The metal shells are made by shallow stamping or deep stamping of metal sheets with threading or pleated flare processing added as required. Specifically metal caps include items such as crowns, screw caps, pilfer-proof caps and twist-off caps. The metal shell portion of the caps may be uncoated, may be treated with an anti-corrosion coating, and may have printing applied. Shell materials suited for use in the present invention include both non-magnetic metal materials such as aluminum, aluminum alloy, copper and brass and magnetic metal materials such as tin plate, tin free steel and coated steel plate.
In order to prevent the seal lining layer used in a cap from separating from the cap during transport or during supply to cap supply chutes in packing and sealing apparatus, the lining layer is ordinarily adhered to the entire inner surface of the bottom of the metal shell by means of a primer layer. However, with the recent appearance of caps, and particularly crowns, used as prizes, there have been various proposals for metal caps in which the lining layers are easily peelable from the metal shells. Prize marketing caps include those having a winning ticket for a prize or other characters, codes or drawings printed as marks on the inside bottom surfaces of metal caps with the idea of promoting the marketing of the bottles. In order to make the peeling of the lining layer easy in this type of prize cap, only the center part of the lining layer is lightly affixed to the metal cap leaving all or part of the outer periphery of the lining layer in a non-adhesive state. For example, the method of manufacture of some caps includes the step of having adhesive primer painted only in the center part of the bottom inside surface of the metal shells, while other methods included having the entire bottom inside surface of the shells painted with adhesive primer, and then having ink that will prevent adhesion between the lining layer and adhesive primer painted in part or all of the outer periphery of the bottom surfaces. However, both of these methods require special types of primer painting apparatus adding a step to the manufacturing process and both methods require precise control of the painting process to prevent poor registration.
A problem which exists where a lining layer comprising a resin such as polyethylene is used, is that the lining layer, if it is adhered over the entire bottom surface of a cap, will tend to crack along its outer periphery over a period of time after bottling leading to a loss of sealability. This is in part due to stress cracking characteristics of polyethylene material.
It is therefore an object of the present invention to provide for a metal cap and method of manufacture of the same which does not require any complicated adhesive primer painting process to affix a part of a lining layer to a primer layer over a portion of the inside bottom surface of a metal cap.